A dynamic random access memory (DRAM) that a data is stored in a memory cell capacitor thereof is generally known as one of semiconductor memory devices. A silicon oxide film is conventionally used as an insulating film of the memory cell capacitor.
In recent years, various DRAMs using a ferroelectric material for the insulating film of the capacitor have been developed to realize nonvolatility of stored data (U.S. Pat. No. 4,873,664, for example). As one of such DRAMs, a semiconductor memory device having the structure as described below is known.
This semiconductor memory device comprises, as basic components, a memory cell having a MOS transistor and a ferroelectric capacitor, a pair of bit lines coupled to the memory cell, a word line coupled to the memory cell, a plate line coupled to the memory cell, and a sense amplifier coupled to the pair of bit lines. The MOS transistor of the memory cell is coupled to the bit lines while the ferroelectric capacitor is coupled between the MOS transistor and the plate line.
Writing of data in this semiconductor memory device is performed by the method which consists in applying a reverse logical voltage to the ferroelectric capacitor of the memory cell. Reading of data is made by the method which consists in taking out electric charge stored in the ferroelectric capacitor from the pair of bit lines as potential and amplifying the potential difference between those bit lines with the sense amplifier.
However, with a semiconductor memory device of the conventional construction as described above, the potential difference for reading produced between the bit lines diminishes if the parasitic capacitance value of the bit lines gets smaller. For that reason, it becomes impossible to accurately amplify this potential difference with the sense amplifier, leading to frequent errors in the reading.